[meteorite-list] Strewn fields vs Distribution Ellipses

From: David Freeman <dfreeman_at_meteoritecentral.com>
Date: Thu Apr 22 10:17:49 2004
Message-ID: <3FE09363.8080602_at_fascination.com>

Thank you Elton;
Yup, shotgun.
df

E. L. Jones wrote:

> Hello List,
> Let me throw this out to help clarify the concepts. I have a
> statistical major but after my accident some procedures are fuzzy to
> me and today most is done by computers anyway, so bear with me. I'll
> try too post a field technique for estimating an ellipse another time.
>
> The strewn field is the physical distribution of the meteorites on the
> ground. It's true density, content and numbers are never known because
> you never know if you have recovered the very last meteorite. It
> tends to fit within an elliptical form, but when actually plotted can
> look like a Rorschach figure. It dosen't have a line drawn around
> it--not yet anyway.
>
> The Distribution Ellipse is the geometric/mathmatical representation
> of the "predicted" boundary or"limit" or "edge" of the strewn field.
> The first reason for computing an ellipse is to focus search and
> recovery efforts. Its secondary use is to estimate angle of fall and
> altitude of fragmentation. Its tertiary use is to confuse novice
> meteorite hunters. When all the math and geometry is done you have an
> ellipse to overlay on your map board. The statistical significance of
> this a prediction is that 95%(2Standard Deveations-SD) or 99 %(3SD) of
> all the meteorites in this fall lie within the ellipse.
> For actual use You duplicate the ellipse overlay on tracing paper and
> give it to each hunter team and send them off to cover each sector
> with an agreed upon search pattern( grid walk, etc.), adjusting
> assigned sectors based on the terrain and best judgement of the search
> director who may be playing every position if they are first at the
> scene. It maybe wise to search the centerline first.
>
> As previously mentioned you need a minimum 4 points to start. Actually
> there can be a little cheating as to the number of points if one of
> the points includes what is clearly a large mass it is treated as two
> close points--i.e. The large mass is assumed to have fallen ON axis
> and/or you have additional clues as to the direction of fall. Remember
> this is a recovery tool which is refined as more finds trun up.
>
> That said-- for an ellipse to be of search value you must have good
> sampling from the whole field which you are trying to estimate and
> that can be a catch-22 loop. For practical use, a laptop with a spread
> sheet template or statistical package is the only efficent way to get
> to the 95% confidence level and add additional locations. The inital
> plot will likely become skewed as more finds are plotted and the
> ellipse has to be adjusted and the new information shared with the
> searchers. It also assumes that you have a simple fall and you do not
> have multiple concentrations due to multiple fragmentations at altitude.
>
> The eccentricity of an ellipse and the length of the two axies give a
> clue as to the angle of the fall and altitude of fragmentation. A
> narrow ellipse suggests a lower level fragmentation because the
> fragments have less time to disperse on the lateral axis before
> reaching the ground. An elongated ellipse suggests a low angle of fall
> (shallow) with fragments dropped along a path where the bulk of the
> mass carried forward momentum longer before reaching the ground.
> Conversely, a wider ellipse suggests a high altitude fragmentation. A
> short ellipse suggests a high overhead (steep) angle of fall.
>
> Conventional wisdom says that larger masses are found at the far end
> of the direction of fall. And perhaps this holds true for a simple
> fall--one that has a single fragmentation episode. However Portales
> Valley had the larger fragments fall to the near end of the field.
> <http://www.lpi.usra.edu/meetings/LPSC99/pdf/1964.pdf>
>
> An example of a complex distribution can be seen at
> <http://www.saharamet.com/meteorite/data/map/ellipse.html> . Note that
> there are several ellipses on the direction of fall suggesting several
> fragmentation episodes. This page shows the initial ellipse from the
> first expedition<http://www.saharamet.com/expedition/CO3/part1.html>
> See how the ellipse plot was expanded as new material was found? The
> actual strewn field recoveries are plotted both inside and outside the
> ellipses where they were recovered over two or more expeditions.
>
> Elton
> (Thank you, Dr. Jean Dyer,USARI, where ever you are)
> mike miller wrote:
>
>> Hello, my question is about "strewnfields" in general, not how they
>> relate to a dry lake bed. I started as a meteorite hunter and have
>> evolved into a hunter/collector.
>> I think I understand the end of a strewnfield, the elliptical shape
>> is created by most of the lighter material falling away to earth and
>> the larger pieces continuing on a path down the middle of the
>> strewnfield. They create a point that more or less follows the middle
>> of the strewnfield. My first question is what is the reason for the
>> point at the beginning of a strewnfield?
>> The second part of my question is are there other types of
>> "strewnfields"? More to the point, explosions that occurred closer to
>> the surface and that might be described as a shotgun pattern? I would
>> be interested in any other types as the only types I have heard about
>> are the classic elliptical and a few related pieces.
>> Thanks in advance for your thoughts and opinions.
>>
>> Mike Miller
>
>
>
>
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Received on Wed 17 Dec 2003 12:33:23 PM PST